linux-kernel - Re: [PATCH] uaccess: Comment that copy to/from inatomic requires page fault disabled

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Message-ID: <20250915164313.42644914@pumpkin>
Date: Mon, 15 Sep 2025 16:43:13 +0100
From: David Laight <david.laight.linux@...il.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Linux Trace Kernel
 <linux-trace-kernel@...r.kernel.org>, Linus Torvalds
 <torvalds@...ux-foundation.org>, linux-mm@...ck.org, Kees Cook
 <keescook@...omium.org>, Aleksa Sarai <cyphar@...har.com>, Al Viro
 <viro@...IV.linux.org.uk>
Subject: Re: [PATCH] uaccess: Comment that copy to/from inatomic requires
 page fault disabled

On Wed, 10 Sep 2025 16:18:20 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> From: Steven Rostedt <rostedt@...dmis.org>
> 
> The functions __copy_from_user_inatomic() and __copy_to_user_inatomic()
> both require that either the user space memory is pinned, or that page
> faults are disabled when they are called. If page faults are not disabled,
> and the memory is not present, the fault handling of reading or writing to
> that memory may cause the kernel to schedule. That would be bad in an
> atomic context.
> 
> Link: https://lore.kernel.org/all/20250819105152.2766363-1-luogengkun@huaweicloud.com/
> 
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> ---
>  include/linux/uaccess.h | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
> index 1beb5b395d81..add99fa9b656 100644
> --- a/include/linux/uaccess.h
> +++ b/include/linux/uaccess.h
> @@ -86,6 +86,12 @@
>   * as usual) and both source and destination can trigger faults.
>   */
>  
> +/*
> + * __copy_from_user_inatomic() is safe to use in an atomic context but
> + * the user space memory must either be pinned in memory, or page faults
> + * must be disabled, otherwise the page fault handling may cause the function
> + * to schedule.
> + */
>  static __always_inline __must_check unsigned long
>  __copy_from_user_inatomic(void *to, const void __user *from, unsigned long n)
>  {
> @@ -124,7 +130,8 @@ __copy_from_user(void *to, const void __user *from, unsigned long n)
>   * Copy data from kernel space to user space.  Caller must check
>   * the specified block with access_ok() before calling this function.
>   * The caller should also make sure he pins the user space address
> - * so that we don't result in page fault and sleep.
> + * or call page_fault_disable() so that we don't result in a page fault
> + * and sleep.

It is worse than that - it must avoid a COW fault as well.
I suspect the comment should really be that these are not the functions you
are looking for, you probably want the 'nofault' variants.

Even if the code thinks it has pinned the user buffer it has to be better
to use the 'nofault' variant.

The only exception might be in code that already has page faults disabled.
But even then it would have to be pretty performance critical for normal code.

	David  


>   */
>  static __always_inline __must_check unsigned long
>  __copy_to_user_inatomic(void __user *to, const void *from, unsigned long n)